r.sim: Parallelize dx/dy derivatives computation using OpenMP

raster/r.sim/r.sim.water/testsuite/test_r_sim_water.py

@@ -13,6 +13,7 @@ class TestRSimWater(TestCase):
     dx = "tmp_dx"
     dy = "tmp_dy"
     depth = "tmp_depth"
+    depth2 = "tmp_depth2"
     discharge = "tmp_discharge"
     diff_depth = "tmp_diff_depth"
     diff_discharge = "tmp_diff_discharge"
@@ -132,6 +133,7 @@ def test_nodxdy(self):
             depth=self.depth,
             discharge=self.discharge,
             random_seed=1,
+            nprocs=1,
         )
 
         # Assert that the output rasters exist
@@ -158,6 +160,21 @@ def test_nodxdy(self):
         stats = tools.r_univar(map=self.diff_discharge, format="json")
         self.assertAlmostEqual(stats["sum"], 0, delta=1e-3)
 
+        # test parallelized dxdy
+        # lower precision because parallelization affects random number generator
+        self.assertModule(
+            "r.sim.water",
+            elevation=self.elevation,
+            depth=self.depth2,
+            random_seed=1,
+            nprocs=2,
+        )
+        self.assertRastersEqual(
+            self.depth2,
+            reference=self.reference_depth_default,
+            precision="0.02",
+        )
+
     def test_complex(self):
         """Test r.sim.water execution with more complex inputs"""
         # Run the r.sim.water simulation

raster/r.sim/simlib/derivatives.c

@@ -20,6 +20,11 @@ void derivatives(const Geometry *geometry, float **elevation, double **dx,
     H = (geometry->stepx / geometry->conv) * 8.0;
     V = (geometry->stepy / geometry->conv) * 8.0;
 
+#if defined(_OPENMP)
+#pragma omp parallel for default(none) schedule(static) \
+    shared(geometry, elevation, dx, dy, H, V)           \
+    private(col, c1, c2, c3, c4, c5, c6, c7, c8, c9)
+#endif
     for (row = 0; row < geometry->my; row++) {
         for (col = 0; col < geometry->mx; col++) {